The present invention relates to a method and apparatus for providing optical switching and, more particularly to a liquid crystal assembly for selectively transmitting colored illumination.
Current optical switching devices typically employ materials which absorb a significant amount of light, resulting in high transmission losses. Improvements in transmissivity have been accomplished by using chiral or cholesteric liquid crystals which have relatively minimal absorptive characteristics. Cholesteric liquid crystals exhibit circular dichroism and can be used to polarize light from a received source, selectively transmitting and reflecting the light, thereby minimizing its attentuation. However, even cholesteric liquid crystals, as conventionally utilized, can involve a transmission loss of at least 50% due to the reflection effects.
The use of phase mirrors and waveplates, interposed between the crystals and the observer to recapture reflected light and to regulate the polarization sense, provides for a zero loss, 100% transmission optical switch. This combination can also be used to provide for a full color selection assembly, which also has the potential for virtually 100% light transmission. Furthermore, since the crystals are optically clear for wavelengths outside of a selected range, 100% of the incident light outside of the narrow spectral range is transmitted. Thus, the transmitted image can be superimposed on an image created by incident light from other sources, having wavelengths outside of the narrow spectral band of a particular crystal.